The subject matter herein relates generally to downhole flow meters, and, more particularly, to an improved downhole flow meter capable of measuring the physical characteristics of a flow comprising more than one phase of matter, for example liquid and gas, also known as a multiphase flow meter.
Flow meters provide critical measurements concerning the characteristics of a flow within a pipe, for example the rate and volume of material flowing through the pipe, as well as pressure and temperature measurements. This is especially true in downhole applications, such as those in which a flowmeter is used to measure material flow in an oil well below the well head. The data produced is used to not only monitor and quantify the well output, but to evaluate overall well conditions and operational performance. Downhole meters must therefore be robust in nature in order to function in the severe environments experienced in downhole applications, for example within widely varying temperature extremes, high flow rates and high pressure, while producing highly accurate measurements in order to properly quantify well production levels and assess operational characteristics.
Several devices are currently used to perform flow measurements in downhole applications. For example, turbine flow meters use a spinning blade that is placed into a flow within a pipe located below a well head. As the material from the well flows past the blade, the blade turns. A linear relationship exists between the rotational speed of the blade and the flow rate, such that the flow rate can be determined from the speed of the rotation. Additionally, each rotation of the blade results in a given volume of fluid passing the blade, thereby also enabling volumetric measurements of the flow. However, because the blade must be free to rotate, it cannot fully occupy the full inner diameter of the pike within which it is placed, resulting in some of the material passing the meter without being measured, also known as slip. The resulting nonlinearity in the volume of material to blade rotation results in inaccuracies in the measurements. Additionally, because a turbine flow meter utilizes a moving blade, it can be susceptible to breakage and maintenance issues, with loose or broken parts being particularly problematic to downstream components in a given well system. Also, a typical flow within a well contains a mixture of liquid and gas components, such as crude oil, water and natural gas, which a turbine flow meter cannot differentiate between. Accordingly, the accuracy of a turbine flow meter may not be sufficient in all applications, such as where separately quantifying the volumetric amount of crude oil and natural gas a well is producing is required.
Other techniques used to measure downhole flow include the use of pressure sensors placed along plugs positioned in the center of a pipe beneath the well head. The plug occupies a portion of the pipe diameter through which the flow travels, thereby causing a disturbance in the flow as the fluid and gas move past. By measuring the pressure in the pipe and the differential pressure around the plug the flow rate can be determined. One advantage to this technique is that it eliminates the need for moving parts within the system. However, the results obtained have less accuracy than those obtained using a turbine flow meter. Additionally, measurement accuracy is dependent on positioning the plug in the center of the pipe, which can be difficult to correctly establish and maintain over time in downhole environments.
It would be advantageous to provide a downhole flow meter that is not only mechanically robust and capable of operating in the severe environment experienced in downhole applications, but which also provides highly accurate measurements of flow characteristics, and which is capable of differentiating between the different phases of matter present in the flow.